Building Design and Construction Handbook - Merritt - Ventech!

13.2.7 Enthalpy
HEATING, VENTILATION, AND AIR CONDITIONING 13.13
Enthalpy is a measure of the total heat (sensible and latent) in a substance and is
equivalent to the sum of its internal energy plus its ability or capacity to perform
work, or PV/J, where P is the pressure of the substance, V its volume, and J its
mechanical equivalent of heat. Specific enthalpy is the heat per unit of weight, Btu/
lb, and is the property used on psychrometric charts and in HVAC calculations.
The specific enthalpy of dry air h a is taken as zero at 0�F. At higher temperatures,
h a is equal to the product of the specific heat, about 0.24, multiplied by the temperature,
�F. (See Table 13.2.)
The specific enthalpy of saturated air h s, which includes the latent heat of
vaporization of the water vapor, is indicated in Table 13.2. The specific enthalpy
of the water vapor or moisture at the air temperature may also be obtained from
Table 13.2 by subtracting h a from h s.
Table 13.2 also lists the humidity ratio of the air at saturation for various temperatures
(weight, lb, of water vapor in saturated air per pound of dry air). In
addition, the specific enthalpy of saturated water vapor h g, Btu/lb, is given in Table
13.2 and represents the sum of the latent heat of vaporization and the specific
enthalpy of water at various temperatures.
The specific enthalpy of unsaturated air is equal to the sensible heat of dry air
at the existing temperature, with the sensible heat at 0�F taken as zero, plus the
product of the humidity ratio of the unsaturated air and h g for the existing temperature.
13.2.8 Cooling by Evaporation
Evaporation of water requires a supply of heat. If there is no external source of
heat, and evaporation occurs, then the water itself must provide the necessary heat
of vaporization. In other words, a portion of the sensible heat in the liquid will be
converted into the latent heat of vaporization. As a result, the temperature of the
liquid remaining will drop. Since no external heat is added or removed by this
process of evaporation, it is called adiabatic cooling.
Human beings are also cooled adiabatically by evaporation of perspiration from
skin surfaces. Similarly, in hot climates with relatively dry air, air conditioning is
provided by the vaporation of water into air. And refrigeration is also accomplished
by the evaporation of a refrigerant.
13.2.9 Heating by Condensation
Many thermal processes occur without addition or subtraction of heat from the
process. Under these conditions, the process is called adiabatic.
When a volume of moist air is cooled, a point will be reached at which further
cooling cannot occur without reaching a fully saturated condition, that is, 100%
saturation or 100% relative humidity. With continued cooling, some of the moisture
condenses and appears as a liquid. The temperature at which condensation occurs
is called the dew point temperature. If no heat is removed by the condensation,
then the latent heat of vaporization of the water vapor will be converted to sensible
heat in the air, with a resultant rise in temperature.